1. Field of the Invention
The invention relates to communication systems in which information from plural digital signal sources is multiplexed into a single channel for transmission and then demultiplexed back into plural digital signals. In particular, the invention relates to such systems in which the signal sources are of nominally the same data rate but not synchronous (i.e. asynchronous), and in which accommodation to a selectable number of signal sources is achieved.
2. Description of Prior Art
The telecommunication industry has standardized multiplex systems accepting two, four and forty-two DS-1 signals, each nominally 1.544 Mb/sec. Six-, eight- and twelve-input multiplexers are also available. This requires a customer with an odd number of DS-1 sources to use the next larger size multiplexer and to tolerate the unused increased bandwidth requirement and decreased reliable range in point-to-point digital radio transmission. The systems, with fixed numbers of channels, require significant equipment replacement when the customer's needs change, restricting the general usefulness of these designs. At the same time, the number of models that must be designed and stocked by the equipment manufacturer to meet the needs of the market is increased, a factor which also reduces the general usefulness of such designs.
While synchronous multiplexers, programmable to a selectable number of sources, are available for lower-speed data communication, none are available for transmission of high-speed asynchronous signals. High-speed transmissions of digital signals greater than a megabit per second are necessarily asynchronous due to the jitter introduced by the digital network. Common causes of this jitter are independent timing sources, multiplex waiting-time jitter and imperfect clock recovery.